RenderPassEditor : Camera visibility and matte columns

[murraystevenson]

This adds new columns to the Render Pass Editor, providing inclusion and exclusion control over camera visibility and holdout mattes for each render pass.

Putting this up in order to perform some user testing with the CI build before we fully commit to these columns and their underlying options.

[johnhaddon]

Thanks Murray! Really great to see another demonstration of how easy it is to slot new columns into the RenderPassEditor.

All feels pretty good to me, but I do have one query about the behaviour : would there be any merit in making cameraExclusions and matteExclusions work more like exclusions, in that they would clobber any included descendants? An exclusion of /chars will currently clobber an inclusion of /chars/robot because it is a pruning operation. But the same camera inclusion/exclusion results in /chars/robot being visible to the camera...

I can see two arguments :

1. The underlying operations are different, so we shouldn't try to match the outward semantics. Pruning visibility (whether by actual pruning or setting scene:visible = false) fundamentally can't be overridden in descendants, whereas camera/matte can. That's the way of things, so the current behaviour is natural.
2. We're defining new higher-level concepts here, and should give them all matching semantics, regardless of the underlying implementation.

I think I'm reasonably on board with 1, but mostly because of our decision to mix the inclusions/exclusions in with any existing attributes from the scene. If we weren't doing that, and were clobbering the scene attributes, I think the consistency of 2 might be better. Perhaps we should at least include an Info panel in the description describing the mechanism whereby this stuff is applied in terms of attributes?

[murraystevenson]

All feels pretty good to me, but I do have one query about the behaviour...

Yeah, it's a tricky one. On the face of it, there's real merit to aligning all the Inclusions and Exclusions columns so they all behave consistently - one concept to grasp and then you're off and running. Though the underlying mechanisms are different, and we may lose out on some of the flexibility that the renderer attributes provide...

For comparison, I've pushed an extra commit that implements the exclusions clobber descendants behaviour. Might be worth getting a bit of user input on this and we can settle on an approach once I'm back from the frozen north.

[murraystevenson]

Merging the non-clobber approach.